Making a light-harvesting antenna from scratch

At Washington University in St. Louis’s Photosynthetic Antenna Research Center (PARC) scientists are exploring native biological photosystems, building hybrids that combine natural and synthetic parts, and building fully synthetic analogs of natural systems.

One team has just succeeded in making a crucial photosystem component -- a light-harvesting antenna -- from scratch. The new antenna is modeled on the chlorosome found in green bacteria.

Chlorosomes are giant assemblies of pigment molecules. Perhaps Nature’s most spectacular light-harvesting antennae, they allow green bacteria to photosynthesize even in the dim light in ocean deeps.

Biological systems that capture the energy in sunlight and convert it to the energy of chemical bonds come in many varieties, but they all have two basic parts: the light harvesting complexes, or antennae, and the reaction center complexes. The antennae consist of many pigment molecules that absorb photons and pass the excitation energy to the reaction centers.

In the reaction centers, the excitation energy sets off a chain of reactions that create ATP, a molecule often called the energy currency of the cell because the energy stored ATP powers most cellular work. Cellular organelles selectively break those bonds in ATP molecules when they need an energy hit for cellular work.

Green bacteria, which live in the lower layers of ponds, lakes and marine environments, and in the surface layers of sediments, have evolved large and efficient light-harvesting antennae very different from those found in plants bathing in sunlight on Earth’s surface.

The antennae consist of highly organized three-dimensional systems of as many as 250,000 pigment molecules that absorb light and funnel the light energy through a pigment/protein complex called a baseplate to a reaction center, where it triggers chemical reactions that ultimately produce ATP.

In plants and algae (and in the baseplate in the green bacteria) photo pigments are bound to protein scaffolds, which space and orient the pigment molecules in such a way that energy is efficiently transferred between them.

But chlorosomes don’t have a protein scaffold. Instead the pigment molecules self -assemble into a structure that supports the rapid migration of excitation energy.

This is intriguing because it suggests chlorosome mimics might be easier to incorporate in the design of solar devices than biomimetics that are made of proteins as well as pigments.

The photosystem in green bacteria consists of a light-harvesting antenna called a chlorosome and a reaction center. The energy of the light the pigments absorb is transferred to the reaction center (red) through a protein-pigment antenna complex called the baseplate (gold). The antenna (green) is made of rod-shaped aggregates of pigment molecules.

Synthetic pigments

The goal of the work described in the latest journal article was to see whether synthesized pigment molecules could be induced to self-assemble. The process by which the pigments align and bond is not well understood.

“The structure of the pigment assemblies in chlorosomes is the subject of intense debate,” Holten says, “and there are several competing models for it.”

Given this uncertainty, the scientists wanted to study many variations of a pigment molecule to see what favored and what blocked assembly.

Up next

Although this project focused on self-assembly, the PARC scientists have already taken the next step toward a practical solar device. “With Pratim Biswas, PhD, the Lucy and Stanley Lopata Professor and chair of the Department of Energy, Environmental & Chemical Engineering, we’ve since demonstrated that we can get the pigments to self-assemble on surfaces, which is the next step in using them to design solar devices,” says Holten.

“We’re not trying to make a more efficient solar cell in the next six months,” Holten cautions. “Our goal instead is to develop fundamental understanding so that we can enable the next generation of more efficient solar powered devices.”

Biomimicry hasn’t always worked. Engineers often point out early flying machines that attempted to mimic birds didn't work and that flying machines stayed aloft only when nventors abandoned biological models and came up with their own designs.

But there is nothing predestined or inevitable about this. As biological knowledge has exploded in the past 50 years, mimicking nature has become a smarter strategy. Biomimetic or biohybrid designs already have solved significant engineering problems in other areas and promise to greatly improve the design of solar powered devices as well.